1. Field of the Invention
The present invention relates to power supplies and, more particularly, to a power supply with reduced power consumption when a load is disconnected from the power supply.
2. Description of the Related Art
Battery chargers commonly accompany portable electronic devices, such as cell phones, laptops, and MP3 players. A battery charger is a power supply that converts the voltage on an AC input, such as a standard AC wall plug, into a DC output voltage. The DC output voltage generated by the power supply is then provided to the portable electronic device to charge the battery, power the device, or both. Two well-known power supplies are a linear power supply and a switched-mode power supply (SMPS).
FIG. 1 shows a circuit diagram that illustrates an example of a prior-art linear power supply 100. As shown in FIG. 1, linear power supply 100 includes an AC input 110 that receives a low-frequency AC (60 Hz), and a low-frequency transformer 112 that has primary windings 112A and secondary windings 112B.
The primary windings 112A are connected to AC input 110 to receive the low-frequency AC, and the secondary windings 112B are spaced-apart from the primary windings 112A. In operation, the low-frequency AC received by the primary windings 112A induces a low-frequency AC on the secondary windings 112B.
The magnitude of the low-frequency AC on the secondary windings 112B can be stepped up (increased) or stepped down (decreased) by varying the ratio of the number of turns in the primary windings 112A to the number of turns in the secondary windings 112B. In addition, transformer 112 provides the remainder of linear power supply 100, along with any attached electronic device, with electrical isolation from AC input 110.
Further, linear power supply 100 includes an output rectifier 114 that is connected to the secondary windings 112B of transformer 112, and a filter 116 that is connected to output rectifier 114. Output rectifier 114 can be implemented with a conventional rectifier circuit, such as a four-diode bridge rectifier that provides full-wave rectification or a single diode rectifier that provides half-wave rectification.
In the FIG. 1 example, output rectifier 114 is implemented as a four-diode bridge rectifier. Filter 116, in turn, is implemented with an LC circuit that includes an inductor L and a capacitor C. In operation, output rectifier 114 converts the low-frequency AC on the secondary windings 112B of transformer 112 into a DC voltage that is smoothed by filter 116 to generate a DC voltage VCC on an output 116G of filter 116.
Linear power supply 100 additionally includes a conductive line 120 that is connected to output 116G of filter 116, and an output connector 122 that is connected to conductive line 120 to receive the DC voltage VCC. Further, as shown in FIG. 1, a load 124, such as a portable electronic device, is connected to or disconnected from output connector 122 when load 232 is connected to or disconnected from linear power supply 100, respectively.
In operation, when output connector 122 is connected to load 124, the DC voltage VCC is input to load 124, causing a current IL to flow down conductive line 120 into load 124 to charge the battery, power the device, or both. When output connector 122 is disconnected from load 124, the current IL no longer flows down conductive line 120.
FIG. 2 shows a circuit diagram that illustrates an example of a prior-art switched-mode power supply (SMPS) 200. As shown in FIG. 2, SMPS 200 includes an AC input 210 that receives a low-frequency AC (e.g., 60 Hz), an input rectifier 212 that is connected to AC input 210, and a filter 214 that is connected to rectifier 212.
Input rectifier 212 can be implemented with a conventional rectification circuit, such as the four-diode bridge rectifier that provides full-wave rectification, or the single diode rectifier that provides half-wave rectification. In the FIG. 2 example, input rectifier 212 is implemented as a four-diode bridge rectifier. Filter 214, in turn, is implemented with a capacitor C. In operation, rectifier 212 converts the low-frequency AC into a DC voltage that is smoothed by filter 214.
As further shown in FIG. 2, SMPS 200 includes a chopper 216 that is connected to the output of filter 214. Chopper 216 can be implemented with a switch, such as a MOSFET. In operation, chopper 216 chops the smoothed DC voltage generated by filter 214 to generate a high-frequency AC (e.g., 10 KHz-100 KHz). (If chopper 216 can be connected to a DC voltage source, rectifier 212 and filter 214 can be omitted.)
SMPS 200 also includes a high-frequency transformer 220 that has primary windings 220A and secondary windings 220B. The primary windings 220A are connected to chopper 216 to receive the high-frequency AC, and the secondary windings 220B are spaced-apart from the primary windings 220A. The high-frequency AC received by the primary windings 220A induces a high-frequency AC on the secondary windings 220A. (A low-frequency transformer is larger than a high-frequency transformer of equivalent performance.)
As before, the magnitude of the high-frequency AC on the secondary windings can be stepped up (increased) or stepped down (decreased) by varying the ratio of the number of turns in the primary windings 220A to the number of turns in the secondary windings 220B. In addition, transformer 220 provides the remainder of SMPS 200, along with any attached electronic device, with electrical isolation from AC input 210.
Further, SMPS 200 includes an output rectifier 222 that is connected to the secondary windings 220B, and a filter 224 that is connected to output rectifier 222. Output rectifier 222 can be implemented with a conventional rectifier circuit, such as the four-diode bridge rectifier or the single diode rectifier. In the FIG. 2 example, output rectifier 222 is implemented as a four-diode bridge rectifier. Filter 224, in turn, is implemented with an LC circuit that includes an inductor L1 and a capacitor C2. In operation, output rectifier 222 converts the high-frequency AC on the secondary windings 220B into a DC voltage that is smoothed by filter 224 to generate a DC voltage VCC on an output 224G of filter 224.
SMPS 200 additionally includes a conductive line 226 that is connected to output 224G of filter 224, and an output connector 230 that is connected to conductive line 226 to receive the DC voltage VCC. Further, as shown in FIG. 2, a load 232, such as a portable electronic device, is connected to or disconnected from output connector 230 when load 232 is connected to or disconnected from SMPS 200, respectively.
In operation, when output connector 230 is connected to load 232, the DC voltage VCC is input to load 232, causing a current IL to flow down conductive line 226 into load 232 to charge the battery, power the device, or both. When output connector 230 is disconnected from load 232, the current IL no longer flows down conductive line 226.
In addition, SMPS 200 includes a chopper controller 234 that is connected to chopper 216 and conductive line 226. In operation, chopper controller 234 controls the switching frequency of chopper 216 (e.g., the rate at which the MOSFET turns on and off to chop up the smoothed DC voltage).
Further, chopper controller 226 compares the DC voltage VCC on conductive line 226 with a reference voltage, which can be internally generated, to determine whether the DC voltage VCC output to load 232 is within a specified limit. When the DC voltage VCC output to load 232 falls outside of the specified limit, chopper controller 234 adjusts the switching frequency of chopper 216 (e.g., the rate at which the MOSFET turns on and off) to generate a DC voltage that falls within the specified limit.
Chopper controller 234 needs power to operate before SMPS 200 can generate power. As a result, SMPS 200 also includes a chopper controller power supply 240 that provides power to operate chopper controller 234. In the FIG. 2 example, chopper controller power supply 240 is implemented with a linear power supply 242 that includes a transformer 242T that is connected to AC input 210, a rectifier 242R that is connected to transformer 242T, and a filter 242F that is connected to rectifier 242R to generate a DC voltage VDD. In operation, chopper controller power supply 240 continuously supplies the DC voltage VDD and a current to chopper controller 234 as long as the AC input 210 receives the low-frequency AC.
Linear power supply 242 is similar to linear power supply 100, except that rectifier 242R is implemented for exemplary purposes as a single diode rectifier, and linear power supply 242 is much smaller than linear power supply 100 because linear power supply 242 need only power chopper controller 234. Further, if the inductive load provided by the primary windings of transformer 242T of linear power supply 242 is too low, SMPS 200 can include a circuit 244 that lies between the primary windings and AC input 210. In the FIG. 2 example, circuit 244 is implemented with a capacitor, but can alternately be implemented with a resistor or a combination of a resistor and a capacitor.
One drawback of power supplies 100 and 200 is that power supplies 100 and 200 both continue to consume power after the load (the portable electronic device) has been disconnected. With linear power supply 100, even though load 124 has been disconnected, the low-frequency AC continues to be supplied to the primary windings 112A of transformer 112. The primary windings 112A of transformer 112, in turn, provide an inductive load which consumes reactive power even though load 124 has been disconnected.
With SMPS 200, even though load 232 has been disconnected, the high-frequency AC continues to be supplied to the primary windings 220A of transformer 220. The primary windings 220A of transformer 220, in turn, provide an inductive load which consumes reactive power even though load 232 has been disconnected.
Although the reactive power consumed by a single power supply when the load has been disconnected is not large, the cumulative affect of many millions of power supplies results in a significant waste of power on a national and global basis. As a result, there is a need for a power supply for portable electronic devices that reduces the power consumed when a load has been disconnected from the power supply.